The invention relates to a single wheel suspension of a rear wheel, the same being steerable, at least minimally, by way of an actuator, of a vehicle having two wheel paths (a double-track vehicle). The swivel bearing (also known as a wheel carrier) of the suspension which carries the wheel is guided by a longitudinal control arm which extends substantially in the longitudinal direction of the vehicle, as well as by two transverse control arms which extend primarily in the transverse direction of the vehicle, with the sweep thereof oriented forward and toward the inside. The two transverse control arms are positioned in different planes when viewed in the longitudinal direction of the vehicle, wherein the ends of the transverse control arms which face away from the swivel bearing are connected either directly or indirectly to the vehicle construction in a manner allowing at least minimal articulation. Reference is hereby made to DE 41 29 643 C2, and particularly to DE 38 27 039 A1, as relevant prior art.
DE 41 29 643 C2 discloses an example of what is also called a central pivot axle among specialists, and this is characterized by an advantageous kinematic behavior, particularly when used in a driven wheel, and is also more advantageous than the known, so-called semi-trailing arm axle. DE 38 27 039 A1 shows one option for making it possible for a wheel guided in such a manner to be minimally steered. In this case, the toe angle of this wheel can be adjusted by use of an actuator which suitably engages with a bearing of the longitudinal control arm, said bearing being positioned on the vehicle construction. This known arrangement requires additional constructed space, not only at the position of the longitudinal control arm bearing on the vehicle construction, but also in the immediate proximity of the longitudinal control arm, for the horizontal articulation thereof (in the transverse direction of the vehicle). Depending on the form of the vehicle construction in this region, such constructed space may not be available. In addition, it is disadvantageous that two actuators are needed to steer the two rear wheels in the case of a vehicle with two wheel paths. This is not only costly and time consuming, but also difficult to realize in terms of control technology.
In the present case, the problem is that of detailing an improved single wheel suspension and/or vehicle axle, while preserving the advantageous fundamental kinematic and elastokinematic properties, as well as essential dimensions.
This solution to this problem is characterized in that the swivel bearing is connected to the longitudinal control arm, by way of two bearings or joints (the bearings or joints may be referred to herein simply as joints) which are positioned at different heights, when viewed in the longitudinal direction of the vehicle, in such an articulated manner that these bearings or joints describe a steering axis which primarily runs perpendicularly when viewed laterally, and which has a negative scrub radius for the wheel. Each transverse control arm is attached to the longitudinal control arm via one rubber mount or at least one joint having at least one minimal degree of rotary freedom. The swivel bearing is further guided by a tie rod, and supported on a soft rubber bearing, said tie rod being connected to an adjuster element of an actuator, the same being arranged substantially in the center of the vehicle and acting for the two rear wheels thereof, wherein the tie rod can be moved substantially in the transverse direction of the vehicle.
According to the invention, in contrast to DE 38 27 039 A1 discussed above, an independent longitudinal axis of rotation is formed by the transverse control arm joints on the side of the wheel, and the additional degree of freedom created in this manner is bound by a tie rod which engages with the wheel carrier (which is now called a swivel bearing), but which functionally is nothing other than a wheel carrier which is able to swivel about this longitudinal axis of rotation with respect to the longitudinal control arm. The now additional tie rod of the first rear wheel of the vehicle is connected to an adjuster element of what below is also called a steering actuator, and the other second rear wheel of the vehicle is likewise connected via a tie rod to an adjuster element of the same (steering) actuator. This adjuster element, or optionally two adjuster elements of the single actuator according to the invention, which is configured for both rear wheels and is arranged substantially centrally, can be a rod which can move in a transverse direction of the vehicle, by way of example, similarly to a gear rack in the front wheel steering of a passenger vehicle. The actuator can be arranged close to the inner articulation of the two transverse control arms on the vehicle body, or close to an axle carrier, or the like, and therefore requires little additional constructed space.
As mentioned above, the swivel bearing has one degree of rotary freedom, substantially about the vertical axis, with respect to the longitudinal control arm. This degree of rotary freedom can be realized by a hinge joint or by two ball joints, or by equivalent mounts or joints. An adjusting movement of the actuator leads via the tie rod to a rotation of the swivel bearing about the longitudinal axis of rotation, and therefore to a steering angle applied to the wheel which itself is rotatably mounted about its axis of rotation on the swivel bearing. By way of example, the bolting of the two outer transverse control arm bearings can be utilized for constructing said longitudinal axis of rotation, for example by attaching two ball joints, which define this longitudinal axis of rotation, onto the longitudinal control arm, as shown in principle in German patent application DE 102011007283.7. These two ball joints or other joints included for the purpose of realizing the longitudinal axis of rotation advantageously sit in front of the two outer transverse control arm bearings, when viewed in the direction of travel of the vehicle, to configure the longitudinal axis of rotation as close to the center of the wheel as possible, thereby producing an effectively small caster distance. As such, the configuration ensures a minimal transverse offset of the wheel contact point during the steering movement.
As is known, a central pivot axle is characterized by a stabilizing behavior when subjected to braking forces acting on the wheel, given a suitable design of the longitudinal control arm bearing positioned on the vehicle body. As a result of the fact that, as described particularly in EP 0 052 153 B1, the longitudinal control arm bearing on the vehicle body has greater stiffness in a first direction which extends to a large degree in the longitudinal direction of the vehicle (typically this is the radial direction of the bearing) than in the direction of the bearing which is perpendicular thereto and which extends to a large degree in the transverse direction of the vehicle (typically the axial direction of the bearing), there is a direction of displacement of the longitudinal control arm bearing point on the vehicle construction, substantially perpendicular to the two transverse control arms and minimally toward the same, when a braking force is applied. As a result, the wheel undergoes a slight toe-in. The sweep of the two transverse control arms oriented forward and inward—chosen for reasons of space—also works against this movement.
So that, at this point, the tie rod does not obstruct this stabilizing behavior of a central pivot axis, it would be desirable to arrange the tie rod as parallel as possible—when viewed from above—to the two transverse control arms; however, for the most part, this is not feasible due to reasons of space. In order to make it possible to realize an advantageous stabilizing behavior of such a steerable central pivot axis using a tie rod, nevertheless, first a relatively soft rubber bearing is included in the functional chain of the tie rod, meaning between the swivel bearing and the tie rod and/or between the tie rod and the adjusting element of the actuator, wherein relatively soft rubber bearings are those which have a stiffness less than 5000 N/mm. Moreover, by realizing a negative scrub radius on a single wheel suspension according to the invention, the configuration ensures that the wheel continues to acquire toe-in when subjected to braking force. This negative scrub radius in this case can be implemented by a suitable arrangement of the joint or the like which determines the longitudinal axis of rotation.
Finally, the tie rod can optionally be arranged with a sweep facing at least slightly forward and inside, even if the magnitude of the sweep, meaning the sweep angle with respect to the transverse direction of the vehicle, will typically be smaller than that of the two transverse control arms. In addition, it has been recognized that a favorable toe-in (=the profile of the toe-in via the wheel hub) can be realized if the tie rod rises from the adjusting element of the actuator to the swivel bearing to a small degree—meaning if the joint between the tie rod and the actuator adjusting element is slightly lower with respect to the driving surface than the opposite joint of the tie rod, wherein the tie rod is connected to the swivel bearing via the opposite end thereof, preferably with a so-called steering arm connected in-between, the same being a component of the swivel bearing and/or rigidly connected to the same.
As already mentioned, the longitudinal control arm bearing positioned on the side of the vehicle construction on typical central pivot axles has higher stiffness in a first direction in which the greater part of the same extends in the longitudinal direction of the vehicle (=typically the radial direction of the bearing) than in the direction perpendicular thereto in which the greater part extends in the transverse direction of the vehicle (=typically the axial direction of the bearing). In the case of typical, non-steerable central pivot axles, the ratio of this stiffness can be in the range of 7:1. In the case of a steerable central pivot axle according to the invention, such flexibility of the longitudinal control arm bearing in the transverse direction of the vehicle is no longer necessary, because the desired “assumption of toe-in” of the wheel when subjected to braking force at this point is realized by the negative scrub radius. At this point, the longitudinal control arm bearing positioned on the side of the vehicle body can be designed to support the above configuration, while advantageously realizing an increased lateral guidance, to such a degree that the stiffness thereof in the first direction (the radial direction of the bearing) is not more than three times as large as the stiffness of the second direction, the same extending with the larger part thereof in the transverse direction of the vehicle (=the axial direction of the bearing).
In addition, it has been recognized that it is also possible to influence the toe-in curve (=the profile of the toe-in via the wheel hub) in a desired manner via the distance of the outer tie rod articulation point—meaning the point thereof facing opposite the wheel—from the instantaneous axis of a single wheel suspension according to the invention, said instantaneous axis being known to be determined by the longitudinal control arm bearing at the end closest to the vehicle body, as well as the pins of the two transverse control arms. Here, the deviation of the toe-in curve of a similar, but nevertheless non-steerable single wheel suspension must be larger in proportion to how far the instantaneous axis can move away from the outer tie rod articulation point when the wheel compresses and extends the suspension with respect to the vehicle body. It would be desirable to achieve the least possible effect of the tie rod and/or said tie rod articulation point on the toe-in curve, said effect occurring if the outer tie rod articulation point would be positioned in the longitudinal control arm bearing at the end closest to the vehicle construction—which nevertheless is not recommended for multiple reasons, as stated above. In contrast, an arrangement of the tie rod together with the actuator closer to the wheel axis, which is advantageous in regards to a short path of travel for actuator movements, causes deviations from the toe-in curve of a similar, non-steerable single wheel suspension. These deviations, however, can be suitably taken into account during the constructive orientation of the instantaneous axis, such that the resulting toe-in curve once again corresponds to the original design, particularly for a similar, non-steerable single wheel suspension.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.